The production of fine fibers from polymeric solution through electrostatic spinning (a.k.a. “electro-spinning”) via an electric field created by a voltage differential between a collecting electrode and a spinning electrode is known. For example, as shown in U.S. Pat. No. 6,743,273, polymeric solution is pumped to a spinning electrode in the form of a rotating emitter in which the pump solution is pumped from a reservoir and forced through holes in the emitter. Upon exiting, the electrostatic potential between a grid and the emitter imparts a charge which causes the liquid to be “spun” as thin fine fibers where they are collected on a substrate as an efficiency layer. During this process, the solvent is evaporated off the fine fibers which draws down the fiber diameter during their flight.
Another example of an electrostatic spinning device is shown in Patent Publication Nos. US2006/0290031 and WO2006/131081. The spinning electrode designs disclosed in these applications are in the form of a rotating drum-like body that may take several different forms. The drum is situated and bathed within a polymeric solution reservoir and is rotated about an axis perpendicular relative to the path of a collection media. By rotating the drum through the polymer solution, the spinning surface of the charged electrode is coated with the polymeric solution. Various drum like body variations are shown throughout these two patent publications to include providing a multiple pointed tips to create discrete spinning locations where fine fibers are generated.
Additionally, in relation more specifically to fine fibers for filter media, there is U.S. Patent Publication No. 2007/0163217, the entire disclosure of which is hereby incorporated by reference as this application shares a common inventor with the present application. The '217 publication provides filter media cellulosic/polyamide composites with solvent bonding between the fine fiber layer and the substrate layer. As disclosed therein, cellulosic/polyamide composites were produced by way of pumping and forcing polymeric solution through small nozzles for deposition on a cellulosic substrate material with sufficient residual solvent to provide solvent bonding to prevent delamination between the substrate and fine fiber layer. Such delamination or loss of part of the fine fiber layer can undesirably change filtration characteristics. According to examples therein, fine fibers made from a polyamide were made in fiber sizes of 120 nanometers, 300 nanometers and 700 nanometers according to methodology explained therein. The polyamide and cellulosic filter media composite material disclosed the '217 publication affords certain filtration characteristics as shown by a pore size distribution histogram being shown therein. The present application and invention relates to developments and advances in the state of the art.